The present invention relates generally to an acoustic well logging method and system for examining the earth's subsurface formations surrounding a borehole. More particularly, it relates to an improved method and system for acoustic well logging to obtain an indirect measure of the shear wave velocity of formations surrounding the borehole, especially in "slow" formations.
It is a well known practice to survey formations adjacent to and surrounding well boreholes by acoustic logging techniques in which acoustic signals are generated and received by means of a logging tool run through the well borehole. One acoustic well logging technique involves the generation and reception of pressure waves and the determination of the instant when the amplitude of the detected pressure wave exceeds a prescribed threshold. This technique generally depends upon the generation and reception of pressure waves and a determination of the traveltime of the pressure waves between the source and the receiver or between spaced receivers. The recorded time provides a measure of the compressional wave velocity in the earth's formation surrounding the borehole. A conventional well logging system generally includes a logging sonde suitable to be suspended in the borehole, a source with the sonde for generating pressure waves in the borehole fluid and one or more receivers with the sonde and spaced apart from the source for detecting pressure waves in the borehole fluid. The pressure waves in the borehole fluid generated by the source are refracted into the earth's formation surrounding the borehole. They then propagate through a portion of the formations, are refracted back into the borehole fluid at a point adjacent to the receivers and are then detected. The ratio of the distance between the source and receivers to the time between the generation and detection of the pressure wave can yield the compressional wave velocity of the formation.
With reference now to FIG. 1, when the pressure waves generated in the borehole fluid reach the borehole wall, they can produce refracted compressional waves in the surrounding earth formations as described above. In addition, the pressure waves can also produce a refracted shear waves in the surrounding earth formations, as well as guided waves, such as Stoneley waves, which travel partly in the borehole fluid and partly in a formation adjacent the borehole. Part of the shear waves are refracted back into the borehole fluid in the form of pressure waves which are detected by the receiver with the sonde. Guided waves are similarly detected by the receivers only delayed in time. In addition, part of the pressure waves never enter the formations but rather propagate in the borehole fluid as fluid waves directly to the receivers. The fluid waves generally travel at the velocity of sound in the borehole fluid but are of relatively low amplitudes and high frequencies, which can often be lost in the recorded signals. Any wave which is one of the four types of waves detected by the receivers can be called an arrival; the pressure wave in the borehole fluid which causes the refraction of compressional waves in the formations, the compressional wave arrival; those caused by refraction of the shear waves in the formation, the shear wave arrivals; those caused by guided waves, the guided wave arrivals; and those caused by fluid waves, the fluid wave arrivals. Thus, the signals generated by the receivers are composite signals which include the compressional wave arrivals, the shear wave arrivals, the guided wave arrivals, and the fluid wave arrivals. In earth formations, compressional waves travel faster than shear waves, and shear waves in a formation usually travel faster than the guided waves. Therefore, the composite signals generated by the receivers include the compressional wave arrivals as the first arrivals, the shear wave arrivals generally are the second arrivals, and the guided wave arrivals as the last arrivals. However, in certain situations refracted shear wave velocities are impossible or difficult to measure, for example, in "slow" formations where the shear wave velocities of the formations are less than or equal to the pressure wave velocity in the borehole fluid, or because the shear wave may not propagate through the formation.
In conventional pressure wave logging apparatus, the relative amplitudes of the refracted shear and compressional waves are such that it is difficult to distinguish the latter shear wave arrival from the earlier compressional wave arrival and from the reverberations of the, borehole caused by the refraction of the pressure wave in the formation. Therefore, it is difficult to use conventional logging systems for logging shear wave velocities. More recently, full wavetrain acoustic well logging systems have been developed to obtain a measure of formation shear and compressional wave velocities as shown by Parks, et al., in U.S. Pat. No. 4,562,557 and Ingram, et al., in U.S. Pat. No. 4,575,830. Parks and Ingram describe a method for estimating or determining the velocities of various arrivals in composite signals recorded by a linear array of receivers. The method of determining or estimating velocities of these arrivals in the composite signals generally comprises the steps of generating acoustic energy in the borehole and recording a signal waveform representative of acoustic energy received at a plurality of points spaced vertically along the length of the borehole after refraction, reflection and direct transmission through and along the formations adjacent to the borehole. A window is established having a predetermined length and moveout. This window is positioned along the composite signal at estimated arrival times. The energy of the signal recorded in this window is multiplied at each point by the window. A Fourier transform of the multiplied energy is obtained to produce a plurality of complex signals in the frequency domain which are analyzed to obtain an estimate of the arrival velocities. The foregoing technique operates where the complex signal indeed includes shear wave arrivals. There are, however, other formation conditions that severely attenuate the propagation of the shear wave or instances where shear wave will not propagate. Hence, Ingram and others have suggested techniques for indirectly obtaining a measure of formation shear wave velocities from determined Stoneley wave velocities data. However, none of these techniqes actually determine measures of the formation and borehole fluid properties at selected locations of interest in the borehole during the course of logging necessary for constraining the inversion of the Stoneley wave velocity data to obtain formation shear wave velocities. Rather, these techniques depend on iterative methods using: lookup tables having ranges of anticipated parameters; estimated parameters; or measures of parameters obtained exterior from the borehole itself.
The present invention provides a solution to the need to obtain the measure of shear wave velocities in formations where it is difficult or impossible to obtain such measure directly. Specifically, the present invention provides an apparatus a method and system for obtaining a measure of formation and borehole fluid parameters at selected locations of interest in the borehole during the course of acoustic well logging so as to provide a more accurate indirect measure of shear wave velocities by inversion of Stoneley wave velocities and does so substantially concurrent with the acoustic well logging process.